Frequency comparison of blast-induced vibration per delay for the full-face millisecond delay blasting in underground opening excavation

Responses and damage to the structures subjected to blast-induced vibration are highly dependent on vibration frequency. Understanding the vibration frequency characteristics and its formation mechanism is essential for the safe and economic design of mining and construction blasts. When millisecond delay blasting is used in excavation, a global frequency value of all of the delays of vibrations fails to present a true picture of the vibration frequency characteristics. In the present study, comparisons of frequency characteristics for the blast-induced vibration per delay were first conducted via an underground opening case excavated by the full-face millisecond delay blasting sequence. The results show that if the blasthole geometry and charge structure are kept the same in each blast delay, the frequencies of single-delay vibration signals will decrease as the dimensions of the equivalent blasting vibration sources increase. However, the cutting blast used for the headmost holes inside is an exception, and its vibration frequency is lower than that of the breaking blast outside. It is the authors’ belief that this counter-intuitive phenomenon is caused by the free faces created by the cutting blast for the breaking blast. A small-scale blasting test and the related numerical simulation were subsequently performed to demonstrate this belief. It is found that the vibration frequency from the holes in the free-face blast is higher than that in the confined blast. In terms vibration frequency, therefore, the vibration from the breaking blast is less harmful to structures compared to the cutting blast for a certain velocity, and decreasing the burden of breaking blastholes is beneficial to reduce the vibrational damage of structures.     

Ex-situ probing of PEFC liquid droplet dynamics in the presence of vibration

PEFC performance degradation when exposed to vibration has been attributed to liquid water redistribution; however, fundamental liquid droplet behavior under such conditions is not well understood. As this technology continues to penetrate automotive, aerospace, and marine markets, where vibration and mechanical shock may be common, it is necessary to characterize droplet dynamics under such conditions. This work uses ex-situ experimental techniques to study the effects of sinusoidal vibration on water and gas-diffusion layer interactions for open cathode and flow channel based designs. Parameters such as contact angle, key droplet dimensions, adhesion force, and detachment velocity were measured for a range of frequencies and amplitudes. Vertical and horizontal oriented cells were investigated. The results show that vibration can significantly impact wetting; in some cases, increasing the barrier for liquid removal. Sessile droplet contact angle and dimensions such as height and wetting diameter, shifted toward more hydrophilic wetting under elevated frequency. Droplet detachment velocities were measured to be higher at 100 Hz then at 0 Hz for air speeds up to 10 m s^?1. These results may be of interest to PEFC modelers and flow-field designers.     

Temperature-dependent dissolution and diffusion of H isotopes in iron for nuclear energy applications: First-principles and vibration spectrum predictions

The dissolution and diffusion of H isotopes in bcc-Fe are fundamental and essential parameters in the H energy application from nuclear conversion, whereas the relevant data are lacking and relatively dispersive, demonstrating some important factors have been missed during the past studies. Here, we carry out first-principles total energy and vibration spectrum calculations to investigate systematically the interstitial H dissolution and diffusion behaviors in bcc-Fe by considering the temperature effect. Temperature and H chemical potential are two important factors to affect the H dissolution property. In the interstitial lattice, the H dissolution energy referring to the static/temperature-dependent H chemical potential decreases/increases with the increasing temperature. The diffusion activation energy and pre-factor of H also depend on the temperature and increase significantly with the increasing temperature from 300 to 1000 K. The temperature-correction can give a reasonable interpretation for the broad disseminating in the experimental data of H diffusivity in bcc-Fe. Our currently calculated results reveal that phonon vibration energy plays a crucial role in the H dissolution and diffusion with the increasing temperature.     

An application of adaptive techniques to vibration rejection in adaptive optics systems

In modern Adaptive Optics (AO) systems, lightly damped sinusoidal oscillations resulting from telescope structural vibrations have a significant deleterious impact on the quality of the image collected at the detector plane. These oscillations can be observed in any mode of a generic modal representation of the AO wave-front sensor. A technique for the rejection of periodic disturbances recently presented in the literature has been adapted to the problem of rejecting vibrations in AO loops. The proposed methodology aims at estimating the harmonic disturbance together with the response of the plant at the vibration frequency. The algorithm has been tested in simulation on realistic scenarios and at the telescope.     

Tool condition monitoring using K-star algorithm

Cutting tools are required for day to day activities in manufacturing. Continuous machining operations lead tool to undergo wear. Worn out tools effect surface finish during machining. The dimensional accuracy of components is also compromised. Robust tool health is vital for better productivity. Hence, an online system condition monitoring of tools is the need of hour, promising reduction in maintenance cost with a greater productivity saving both time and money. This paper presents the classification performance of K-star algorithm. A set of statistical features extracted from vibration signals (good and faulty conditions) form the input to algorithm. In the present study, the K-star algorithm is able to achieve 78% classification accuracy.     

A realtime curvature-smooth interpolation scheme and motion planning for CNC machining of short line segments

G01 codes generated by CAM (Computer Aided Manufacturing) system are the most common form of tool path in CNC (Computer Numerical Control) machining. For the piecewise linear path, tangential and curvature discontinuities bring about large fluctuation of feedrate and acceleration, which produces vibration of machine tool. In recent studies, the methods for G² (curvature-continuous) tool-path smoothing and jerk-limiting feedrate scheduling were developed. However there still exist the deficiencies when these methods are employed in CNC machining. It is difficult to simultaneously ensure that the tool path is chord-error-constrained and G01-point-interpolated in real time. In addition, heavy computational load hinders realtime processing in CNC system. Recently the scholars experimentally found the potential of G³ (curvature-smooth) trajectory and jerk-continuous motion in reducing the vibration of machinery. This work proposes a realtime tool-path smoothing algorithm, generating G³ interpolative tool path composed by mixed linear and quartic Bezier segments. The purpose of the smoothing scheme is the simultaneous considerations of G³ continuity, confined chord error, G01 points interpolated, and realtime performance. And the tool path generated is optimized in curvature variation energy (CVE) and analytical curvature extrema is available. To reduce the vibration, a high-efficient algorithm of jerk-continuous (JC) feedrate scheduling for G³ tool path is provided. Finally, a realtime tool-path processing scheme is developed, including G³ interpolation and motion planning functions. As shown in the simulation, the contour error, curvature of tool path, feedrate fluctuation and machining time are reduced compared with G² transition scheme. The experiment on a machine tool is conducted to demonstrate the advantages of the proposed algorithm in vibration reduction and surface quality, compared with G² transition scheme.     

Dynamic behavior of thin and thick cracked nanobeams incorporating surface effects

Free transverse vibration of cracked nanobeams is investigated in the presence of the surface effects. Two nanobeam types, thin and thick, are studied using two beam theories, Euler–Bernoulli and Timoshenko. The influences of crack severity and position, surface density, rotary inertia and shear deformation, nanobeam dimension, mode number, satisfying balance condition between the surface layers and the bulk, boundary conditions and satisfying compatibility and boundary conditions with appropriate resultant moment and shear force are studied in details. It is found out that satisfying compatibility and boundary conditions with the resultant moment and shear force in presence of the surface effects and considering surface density neglected in previous work have significant effects on the natural frequencies of cracked nanobeams. In addition, rotary inertia and shear deformation cause a reduction in the crack and surface effects on the natural frequencies.     

城市复杂环境下基坑爆破降振及振动监测

以某市地下人防工程基坑爆破为例,介绍了城市复杂环境下基坑爆破开挖技术。通过优化爆破网路、布置减振孔、设置减振爆区等减振措施成功控制了爆破振动对周边被保护建筑的危害。爆破振动监测结果表明,对于高层框架式结构建筑物,爆破振动信号中的高频成分比低频成分更易于衰减;并且楼房顶层振动速度、振动持时及主频率等特征参数均低于底层,可以为相关爆破工程提供参考。     

Effects of local damage on vibration characteristics of composite pyramidal truss core sandwich structure

The effects of local damage on the natural frequencies and the corresponding vibration modes of composite pyramidal truss core sandwich structures are studied in the present paper. Hot press molding method is used to fabricate intact and damaged pyramidal truss core sandwich structures, and modal testing is carried out to obtain their natural frequencies. A FEM model is also constructed to investigate their vibration characteristics numerically. It is found that the calculated natural frequencies are in relatively good agreement with the measured results. By using the experimentally validated FEM model, a series of numerical analyses are conducted to further explore the effects of damage extent, damage location, damage form on the vibration characteristics of composite pyramidal truss core sandwich structures as well as the influence of boundary conditions. The conclusion derived from this study is expected to be useful for analyzing practical problems related to structural health monitoring of composite lattice sandwich structures.     

Effect of viscoelastic interface on three-dimensional static and vibration behavior of laminated composite plate

In this study, static and free vibration analysis of laminated cross-ply rectangular plate with special emphasis on incorporating viscoelastic interface is investigated using three-dimensional theory of elasticity. The laminated plate is assumed to be simply-supported at four edges and is subjected to uniform pressure at the top surface. State space technique is used along the plate thickness to investigate the space dependent behavior where as time dependent behavior can be discussed by solving first order differential equation of sliding displacement at the viscoelastic interfaces. Numerical results depicts that the present method converges rapidly and good agreement is exist between the present results and the published results. Moreover, the effects of elastic and viscous interfaces, time, aspect ratio and length to thickness ratio on the bending and vibration behavior of laminated plate are studied.